1. Field of the Invention
This invention is in the field of scraping devices used to remove debris from the inner surface of oil field tubulars, such as a casing in an oil well; specifically, this invention is in the field of devices which can remove such debris without rotating the scraping elements relative to the casing, commonly called non-rotating scrapers.
2. Background Art
Casing scrapers have been used for many years to remove scale, perforations, mud cake, cement, or other forms of debris from the internal surface of casing in an oil well. It is particularly important to clean the casing where completion equipment is to be installed. Several companies have provided rotating casing scrapers for many years. Most of these scrapers utilize scraper blades that are pushed against the wall of the casing by compression springs. On most of these scrapers the blades are held in place by a threaded retainer or captured by retaining bolts. If the fasteners vibrate loose or the retainer unthreads, the scraper blades can be left in the oil well. This creates an expensive fishing job to recover the lost blades.
In most applications, the casing scraper is run in the well and possibly rotated for only a short period of time. However, if the wellbore fluid needs to be changed, it may be necessary to rotate the drill string for a long period of time, such as 2 or more hours, during changing of fluid. This is because the ability to rotate the drill string provides the operator with a quicker method to change the wellbore fluid. Since the scraper blades are pushed against the ID of the casing by springs, if the blades are rotated for a long period of time, the casing ID may become worn. So, if a rotating scraper is used, it can be impossible to rotate the drill string for a long period of time while changing the fluid, without damaging the internal surface of the casing. Operators in the North Sea often use a non-rotating casing scraper, because of this problem.
One known non-rotating scraper consists of a non-rotating cylinder that has a larger diameter than the ID of the casing. The cylinder has long axial slits that allow the cylinder to be compressed to fit into the casing. The cylinder can be fitted with blade or brush elements to clean the wall of the casing, as the cylinder is moved through the casing. This type of scraper is very expensive to manufacture and service.
A common disadvantage of non-rotating scrapers is that, if one becomes stuck in a well, it can be difficult to recover. The ability to convert a scraper from non-rotating to rotating could eliminate this disadvantage.